combustion synthesis

Combustion synthesis makes a ceramic by literally setting the ingredients on fire and letting the reaction cook itself. You mix reactants that release a great deal of heat when they combine, ignite one spot with a match-like spark, and a glowing reaction front sweeps through the whole mass like a lit fuse, the heat it gives off igniting the material just ahead of it. Because the reaction fuels its own advance, no furnace is needed once it starts; the process is called self-propagating for exactly this reason, and it can be over in seconds.

There are two common flavours. In self-propagating high-temperature synthesis, or SHS, you press together elemental powders that react fiercely, the classic case being titanium plus carbon igniting to form titanium carbide, Ti + C giving TiC, with the front reaching well over 2000 degrees C on its own. In solution combustion synthesis you dissolve metal nitrates, which act as an oxidiser, together with an organic fuel such as glycine or urea, dry the mix to a gel, and ignite it; the fuel and nitrate burn together in a brief, self-sustaining flame that leaves behind a fine oxide powder. The furious release of gas, water vapour, nitrogen, carbon dioxide, puffs the product into a light, foamy, high-surface-area mass full of pores, ideal for grinding into a fine reactive powder.

The appeal is speed, simplicity, and low external energy: you supply a spark, and the chemistry supplies the heat, so a batch can be made in seconds instead of hours of firing. It suits refractory carbides, borides, nitrides, and many oxide powders, and the foamy product is easy to mill. The honest caveats: the reaction is extremely fast and very hot, so it is hard to control precisely, the product can be non-uniform or incompletely reacted if the front moves too fast, and only reactant systems that release enough heat to keep themselves burning will work at all. It is a spectacular tool, but a blunt one compared with the fine control of sol-gel or hydrothermal routes.